Fuel cells generate electrical energy from chemical reactants by facilitating paired oxidization and reduction reactions, where the oxidization reaction liberates electrons and the reduction reaction binds electrons. The electron donor and electron acceptor are separated (often by an electrolyte, which may be an alkali, molten carbonate, phosphoric acid, proton exchange membrane (PEM), solid oxide, or other materials, and in some cases separation is just by distance), and the fuel cell is arranged such that the electrons from the electron donor must flow through a wire to the electron acceptors. In doing so, they generate an electrical current that can be used to power devices
While various chemical reactants can be employed, for purposes of discussion, the reactant which releases electrons in an oxidization reaction is referred to herein as a “fuel”, and the reactant which gains the electrons is referred to herein as an “oxidizer” (even though it may not actually include oxygen).
Many fuel cell reactions could be generalized, for example, as:Fuel→oxidized fuel++e−Oxidizer+e−→Reduced OxidizerOne common example of a fuel and oxidizer pair are hydrogen and oxygen gasses; in this example, the reactions are:H2→2H++2e−½O2+2H++2e−→H2OThe electrical current results from the fact that the electrons released from the H2 are more energetic (i.e., at a higher potential) than those required to form the H2O.
In a fuel cell, the oxidation reaction occurs at an anode of the fuel cell, and the reduction reaction takes place at a cathode. Electrons flow from the anode to the cathode, and thus through a load placed therebetween, providing direct electrical energy to power the load. The fuel cell is designed to facilitate the oxidation and reduction reactions, typically by employing catalysts. For example, platinum is often used to split H2 into 2H++2e−, although many different catalysts are available, including various metals and alloys (e.g., palladium, iridium, nickel, and combinations thereof), and doped carbon nanotubes (e.g., doped with one of the catalytic metals, and also combinations such as cobalt, nitrogen, and cerium oxide). Catalysts also sometimes incorporate an ionic liquid.
A variety of fuel cells have been proposed and developed, using various fuel and oxidizer reactants, various electrolytes between the anode and cathode, and various approaches to optimize performance for particular situations. Examples fuel cell types include polymer electrolyte membrane (PEM) fuel cells (also referred to as Proton Exchange Membrane fuel cells), direct methanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, reversible fuel cells, microbial fuel cells, and glucose fuel cells. (“Types of Fuel Cells,” U.S Department of Energy, 2014); (“Microbial fuel cell,” Wikipedia, The Free Encyclopedia, 2019); (Ho, “Glucose Fuel Cells,” Stanford university, 2014) Some types of fuel cells operate at very high temperatures (e.g., molten carbonate fuel cells operate at 600 C and above), but some can operate at much lower temperatures (e.g., body temperature, room temperature, or lower).
For powering small devices, it is desirable to employ fuel cells that are simple in structure to facilitate small-scale fabrication. Additionally, when used to power medical or veterinary devices to be employed inside a biological organism, fuel cells should not only be small, but also designed to operate at temperatures typically found in living organisms, and should preferably employ reactants of low toxicity and, at least if they are to be released into the environment, produce waste products of low toxicity (although acceptable toxicity may be relative to the need and/or alternatives; for example, chemotherapy agents used for cancer treatment are quite toxic relative to many other drugs, but when the alternative is death from cancer, this level of toxicity is considered acceptable). Three examples of fuel cells that meet these criteria are hydrogen-oxygen fuel cells, acetylene-oxygen fuel cells, and glucose-oxygen fuel cells. Note that the operation of fuel cells inside living organisms or fuel cells used as part of a device that operates on living organisms (e.g., a cell-based in vitro assay system) is referred to herein as “medical” regardless of the use, or the species of organism.
Hydrogen-oxygen PEM fuel cells are simple and durable in structure, having a proton-conducting polymer membrane positioned between the anode and the cathode. The membrane allows hydrogen ions (protons) to pass through, but blocks passage of electrons. The simple structure facilitates small-scale fabrication, with a functional 1 mm×3 mm×3 mm fuel cell having been fabricated and tested in 2008. (Moghaddam, Pengwang et al., “Millimeter-Scale Fuel Cell With Onboard Fuel and Passive Control System,” Journal of Microelectromechanical Systems, 6, 2008) While simple, a limitation of such PEM fuel cells is that they must be fueled by very pure hydrogen sources, and even tiny amounts of certain contaminants can poison the fuel cell (for example, platinum and platinum alloy catalysts are readily poisoned by small amounts of carbon monoxide). The requirement of providing a source of hydrogen either limits the use to applications where a continual supply of hydrogen is available, generally either via connection to an external hydrogen store (which then requires some form of tubing connected to the fuel cell), or an internal hydrogen store (which is then space-limited, and for small devices, this limitation can be severe), although one example has been proposed where the hydrogen is absorbed directly from the gas chamber of an airship using hydrogen as the lifting gas. (U.S. Pat. No. 6,986,222 (Dossas and Kraft, “Hydrogen Lighter-Than-Air Ship,” U.S. Pat. No. 6,896,222, 2005)) One advantage of hydrogen-oxygen fuel cells for medical applications is that the waste product, water, is non-toxic and thus can be freely released into the organism.
Acetylene-oxygen fuel cells have been used as detectors for sensing the presence of acetylene in dielectric fluid, as taught in U.S. Pat. Nos. 6,436,257 and 8,002,957, both incorporated herein by reference. (Babas-Dornea and Noirhomme, “Means for Detecting and Measuring the Concentration of Acetylene Dissolved in a Fluid,” U.S. Pat. No. 6,436,257, 2002); (Grincourt and Babes-Dornea, “Sensor Apparatus for Measuring and Detecting Acetylene and Hydrogen Dissolved in Fluids,” U.S. Pat. No. 8,002,957, 2011) The '957 patent also teaches a hydrogen-oxygen fuel cell for detecting the presence of hydrogen, and similar use of a hydrogen fuel cell is taught in U.S. Pat. No. 4,293,399 (Belanger and Missout, “Device for Detecting and Measuring the Concentration of Gaseous Hydrogen Dissolved in a Fluid,” U.S. Pat. No. 4,293,399, 1981), also incorporated by reference.
Glucose fuel cells are typically more complex that the fuel cells discussed above, as they typically require an additional chemical reaction to obtain hydrogen from the glucose molecule, the hydrogen subsequently being used as fuel. While more complicated, glucose fuel cells have particular interest for medical applications, as they have potential to operate for extended periods of time in a living organism because both the fuel (glucose) and the oxidizer (oxygen) may be available.